Hydrodynamic drag forces and vortex-induced vibration (“VIV”) are consequences of flow separation, which is, in turn, a result of boundary-layer separation. Boundary layer separation refers to the inability of an energy-depleted boundary-layer to resist an adverse pressure gradient and therefore to succumb to back flow. As the pressure gradients on the surface of a circular cylinder (for example, a drilling riser) in a current are severe, the boundary layer usually separates from the surface at the first encounter with an increasing pressure, usually near points about 90 degrees from the onset current direction. Surface roughness can worsen this behavior.
With a separated flow, the pressure distribution is changed so that the pressure on the downstream side of the drilling riser is more uniform and close to that of the ambient, whereas the pressure on the upstream side is higher—reaching the impact pressure of the current velocity, as shown, for example, in FIG. 2. The result is a pressure difference between the up- and downstream faces of the cylinder which yields a streamwise force termed the pressure drag or the form drag. The actual frictional drag on the attached-flow upstream side is but a very small part of the form drag.
A shear force exists between the separated region of low-velocity flow and the exterior, current-velocity flow. This is manifest by surfaces of vorticity on both sides of the slow wake region. Unfortunately, this vorticity is in unstable equilibrium if distributed uniformly. Rather, the vorticity becomes organized into a stable configuration of discrete vortices of opposite sense and alternating position across the wake. The continued generation of these alternating vortices is accompanied by a corresponding fluctuation in the circulation (integrated peripheral velocity) about the cylinder. This alternating circulation, when combined with the average onset current velocity yields strong alternating lateral lift forces (and some drag forces) on the cylinder. The vibration which may result is vortex-induced, i.e., VIV.
Therefore, VIV is associated with form drag, which is the result of boundary-layer separation. If the form drag can be significantly reduced or eliminated, VIV will be substantially eliminated as well.
According to Bernoulli's equation, the ability of a flow to overcome an adverse pressure gradient—one that tends to retard the flow—is balanced by the inherent kinetic energy of that flow, in ideal (potential) flows. In real flows with inherent viscous loss mechanisms, it seems that with thermodynamic certainty there is insufficient energy available in the originating flow to overcome such losses.
In the past, passive flow control has been used in an effort to prevent flow separation at the boundary-layer in marine bodies, such as drilling platforms and marine risers. An example of a prior art passive flow control system is shown in FIGS. 1A and 1B, which depict a marine riser 100. Marine risers 100 are used, for example, to connect a floating drilling vessel 102 to the ocean floor 104 and to provide a conduit for a drill string and drilling fluids. When beset by ocean currents 106, marine riser 100 will exhibit substantial hydrodynamic drag forces and VIV. Such forces and motions induce mechanical stresses in, and deflections of, the marine riser 100 and its connection 110 to the drilling vessel 102 and connection 112 to the ocean floor 104, which ultimately may result in failure or interference with drilling operations.
Drag and VIV have been reduced by the application of fairings 114 to the marine riser 100. The fairings 114 are enabled passively to rotate about the riser 100 in order to align with the direction of the current 106 to minimize drag. While some drag and VIV reduction is thereby obtained, the procedure for applying and removing fairing segments from riser joints while they are being run and retrieved is lengthy. Slowed riser deployment and retrieval reduces availability and safety of the drilling rig, with important economic consequences. Fairings 114 suffer another disadvantage, in that fairing sections are bulky, expensive, and subject to damage when being deployed through the ocean surface wave zone.
Other techniques for preventing flow separation apply active means, specifically, energetic boundary-layer control (“BLC”). In one such technique, boundary-layer separation is avoided by removing the boundary-layer using suction. For example, suction has been used on terrestrial or airborne moving bodies (such as automobiles and aircraft) by sucking air into the body interior through pores or slots in the body's skin. With the low energy boundary-layer flow removed, the surface is wetted by the high-energy external flow, which, by Bernoulli's principle, can just overcome an adverse pressure gradient that was created by the body itself.
Suction works well in wind tunnels and has also been applied with some success to aircraft wings, not only to reduce pressure drag and promote laminar flow, but also to achieve high lift. Its use in seawater, however, is confounded by the ubiquity of particulate matter and bio-fouling. The porous or multi-slotted skin also tends to be far too fragile for marine application, and too costly.
Another technique involves moving the body in such a way that its tangential motion can be imparted to the wetted surface itself with a velocity equal to or greater than that of the local external flow. If this is done correctly, then there is no retarding shear, and the flow has no cause to separate. This has been successfully attempted with strategically located imbedded rotating cylinders, for example, or with mechanically driven flat belts. Indeed, if an entire circular cylinder is rotated about its axis in a fluid current we have a Flettner Rotor that generates substantial lift forces by its induced circulation. Such rotational techniques, however, would be difficult and costly to implement on a drilling riser or marine platform.
An alternative technique is to discharge or eject fluid out of the body and into the surrounding atmosphere. This technique has been used in automobiles and aircraft. The inventors recognized the application of this technique in marine applications, devising the system and method described and claimed in U.S. Pat. No. 6,148,751, which is incorporated by reference into this application. In the patented system, water can be energetically discharged from a stationary, submerged surface. From another viewpoint, the shear-retarded boundary layer is re-energized by the discharge of a high velocity jet of fluid at some streamwise location before separation occurs. The jets must discharge fluid at velocities higher than that of the local external flow at the ejection locations. This is done in order for the jets themselves to avoid separation in face of the adverse pressure gradients imposed by that external flow. As described in the referenced patent, thin slot nozzles, for example, may be used to discharge fluid that conforms to the curved wall of a circularly-cylindrical body by means of the Coanda effect. This conforming ejected fluid (called “wall-jets”) will be retarded by shear at the stationary wall, by shear against the slower external flow, and by those pressure gradients. However, the wall-jets are endowed with energy much larger than that of the external flow.
Marine drilling risers beset by ocean currents exhibit unique characteristics, and it would be advantageous to have an active BLC system that conforms to these characteristics. For marine drilling risers, the requirements of compactness and buoyancy, conditions of remoteness, and deployment/retrieval are most demanding. On the other hand, the payoffs for significant reductions of drag and VIV are quite apparent.
Accordingly, the present invention provides an active BLC system designed for use with marine drilling risers. Those skilled in the art, however, will recognize that the present invention is not limited solely to marine drilling risers. There may be other marine applications where the present invention has practical and advantageous application for reducing boundary-layer separation and associated hydrodynamic drag and VIV.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.